1. Field of the Invention
The present invention involves generally a multimode Ultra-High Frequency (UHF) antenna having a steerable cardioid radiation pattern with application to Automatic Direction Finding (ADF) systems and, more specifically, a multimode avionics ADF antenna having two efficient concentric cavity-backed slot radiators with an electronically rotatable cardioid combined radiation pattern.
2. Description of the Related Art
The primary application of our invention is for use as an Automatic Direction Finding (ADF) antenna for the 225-300 MHz Ultra-High Frequency (UHF) communications band. As is well-known in the art, an ADF antenna must provide a non-ambiguous directionality that is either electrically or mechanically steerable and should meet this requirement for both transmit and receive modes. A typical ADF antenna known in the art exhibits a steerable cardioid radiation pattern that is defined according to the formula R=1+cos .beta., where R is the radiation signal sensitivity and .beta. is the direction angle, with respect to the direction of peak radiation signal sensitivity, in which R is measured.
The classical ADF antenna design employs a simple dipole to generate an ambiguous figure-eight pattern (R=.vertline.cos.beta..vertline.). This simple ADF antenna design is well-known in the art but suffers from three distinct disadvantages. Firstly, the directional sensitivity of the simple dipole is ambiguous, which means that the operator is unable to determine whether his target is at 0.degree. or 180.degree. without moving the antenna and making a second measurement. Secondly, the classical dipole protrudes significantly above the ground plane, making it unsuitable for use with aircraft or missiles. Finally, the conventional ADF systems have relatively low gain (as low as 20 dBi in some parts of the band). This low gain is unacceptable for position locating system applications because the ADF reception performance must provide high efficiency to overcome the low transmitter power (100 mW) provided in the typical survival radial set.
There has been a strongly-felt need for an ADF antenna having a low profile suitable for mounting on the airfoil surfaces of missiles and aircraft. Many practitioners in the art have labored to develop low profile airfoil antennas for a variety of applications. For instance, the slot antenna backed by a resonant cavity is well-known in the art and can be mounted flush with an airfoil surface for a broad range of purposes.
The slot antenna is a cavity resonator, energized by a coaxial feeder, that radiates from a slot aperture in the active transmit mode. The field distribution in the slot therefore is dependent on the excitation of higher cavity modes as well as the principle mode (TE.sub.10). To maximize the radiation conductance, the cavity dimensions must be large enough so that the dominant mode is above cutoff. The important design parameter, antenna Q (Quality Factor), is at a minimum when the stored energy in the cavity is only in the dominant mode. The Q limits the inverse voltage-standing-wave-ratio (VSWR) bandwidth product. For a small cavity, Q&gt;3/4.pi..sup.2 V, where V is cavity volume in cubic wavelengths. The cavity resonant frequency can be lowered by using dielectric or ferrite loading of the cavity. A reduction in cavity volume and aperture size results in increased Q, smaller bandwidth, and lower efficiency. This means that a cavity-backed slot antenna having good efficiency and bandwidth requires a larger cavity volume, which increases the space and protrusion requirements in airfoil applications. A cavity-backed slot antenna functions with similarly directionality and efficiency in the passive receive mode.
Accordingly, practitioners in the art have sought to improve the cavity-backed slot antenna by incorporating changes within the cavity and by using novel slot geometries to increase operating bandwidth and antenna efficiency for low cavity volumes. For instance, H. E. King and J. L. Wong disclose a shallow, ridged-cavity, crossed-slot antenna for the 240-400 MHz frequency range (I.E.E.E. Trans. Antennas Propagat., vol. AP-23, no. 5, September 1975, pp. 687-689). King and Wong demonstrated that a ridged-cavity slot antenna is a viable approach to achieving a wideband VSWR response. Their measurements showed that the ridged-cavity slot antenna also exhibits broadband pattern performance, although the radiation pattern characteristics are sensitive to the vehicle configuration when the slot is mounted on an aircraft. King and Wong neither teach nor suggest any method for isolating the antenna pattern from these airframe coupling effects. Indeed, they conclude that accurate pattern and directivity information will require model measurements.
H. Paris Coleman and Billy D. Wright, two researchers at the Naval Research Laboratory, have also considered the problem of flush-mounting an antenna for ADF applications. Coleman and Wright disclose a flush-mounted cavity-backed slot antenna using two concentric annular slots (I.E.E.E. Trans. Antennas Propagat., vol. AP-32, no. 4, April 1984, pp. 412-414). This dual annular slot antenna provides a good front-to-back ratio in a steerable cardioid pattern over a wide range of elevation angles. Unfortunately, the dual annular design, while smaller than other flush-mounted or low-profile transponder antennas, provides an operating bandwidth of only six percent. Also, the antenna is strongly coupled to the airframe so that pattern performance is strongly dependent on ground-plane configuration. Coleman and Wright neither teach nor suggest means for increasing the operating bandwidth or decoupling the dual annular slot radiators from the ground-plane geometry.
The problem of limited bandwidth in low-profile cavity resonators was addressed by H. K. Smith and Paul E. Mayes (I.E.E.E. Trans. Antennas Propagat., vol. AP-35, no. 12, December, 1987, pp. 1473-1476) by stacking two cavity resonators to increase the effective bandwidth. Smith and Mayes suggest stacking resonators with similar resonant frequencies and coupling energy between the cavities by means of carefully-placed slots in the common wall. They propose a dual cavity-backed slot antenna consisting of two D-shaped cavities stacked one above the other, coupled by two slots in the common wall. However, Smith and Mayes neither teach nor suggest methods for applying their stacking and coupling technique to the problems inherent in ADF antennas using steerable cardioid patterns nor do they consider ground-plane decoupling to preserve pattern performance.
The omnidirectional circumferential slot antenna is also well-known in the art. In U.S. Pat. No. 3,739,386, Howard S. Jones, Jr., discloses the use of a plurality of concentric ring radiating elements in a space projectile for telemetry and other applications. Jones, Jr., teaches the use of concentric ring radiators backed by a single resonant cavity as well as a plurality of stacked cavity backed circumferential radiating slot antennas. In U.S. Pat. No. 3,805,266, Robert E. Munson discloses a novel turnstile slot antenna which makes use of a circumferential slot around a prismatic spacecraft body for use in telemetry and command communications. In U.S. Pat. No. 3,810,183, Jack K. Krutsinger, et al., disclose a dual slot antenna assembly including a pair of concentric, radially-spaced cylindrical conductors defining a pair of circumferential slots which are longitudinally spaced one-half wavelength apart. None of these disclosures teach or suggest the use of the omnidirectional circumferential slot radiator in ADF applications requiring a steered directional antenna pattern nor do they teach or consider solutions to the bandwidth and ground-plane coupling problems common to all reduced-size slot antennas.
Other methods have been proposed by practitioners in the art for improving the bandwidth and efficiency of slot radiator antennas at reduced cavity sizes. For instance, U.S. Pat. No. 4,242,685, issued to Gary G. Sanford, discloses a resonant cavity-backed slot radiating antenna that includes an electrically-conducting plate disposed within the cavity, having no contact with internal cavity walls, to effectively lengthen the electrical dimensions of the cavity and thereby reduce the resonant frequency. Sanford's technique provides a more efficient antenna structure and reduces the requisite physical dimensions for operation at a given frequency, but Sanford neither teaches nor suggests isolation methods to minimize the effects of airframe geometry on the antenna radiation pattern. Also, Sanford's technique does little to increase antenna bandwidth.
In U.S. Pat. No. 4,431,998, Kenneth R. Finken discloses an antenna configuration for shaped-conical or uniform hemispheric coverage using circularly-polarized signals. Finken's antenna is a very thin or flush-mounted radiation structure using an array of elements providing circular polarization. Finken's design emphasizes ease of control over pattern shape and neither teaches nor suggests solutions to the problems of pattern distortion from mounting frame coupling and narrow antenna bandwidth.
In U.S. Pat. No. 4,733,245, Michael E. Mussler discloses an electrically-small, cavity-backed slot antenna having an elongated slot disposed around the perimeter of the cavity-backed radiator surface. Mussler teaches the use of several elongated slot configurations, including substantially rectangular, substantially triangular and circular, as means for reducing the requisite physical size of the cavity resonator without shortening the slot length and thereby unduly sacrificing antenna performance. Although Mussler's teachings do reduce cavity resonator size, he neither teaches nor suggests solutions to the problems of pattern distortion from airfoil coupling and narrow antenna bandwidth.
These unresolved problems and deficiencies are clearly felt in the art and are solved by our invention in the manner described below.